This invention relates to polyethylene production and more particularly, it relates to a procedure for effective removal of polyethylene build-up or agglomeration of polyethylene from a Unipol reactor.
Low density polyethylene is the first of the polyolefins prepared by the polymerization of ethylene. Low density polyethylene is a plastic material commonly used in products such as plastic bottles, garbage bags and insulating covers for electrical wires. During the reaction, the reactants are maintained at a temperature of about 250.degree. to 300.degree. F. under high pressure. After formation, the polyethylene is present in small fluidized particles and thus is flowable.
Low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) are produced by the Unipol process developed by Union Carbide. The production of such polyethylene takes place in a low pressure vessel, about 14 feet in diameter and 80 feet high, referred to as a Unipol reactor. The reactor has the problem that sometimes the exothermic reaction will heat up, excessively melting particles of polyethylene, leaving a solidified mass of polyethylene in the reactor. When this happens, the reactor cannot be restarted and thus the mass of solid polyethylene must be removed.
The solidified mass of polyethylene in the Unipol reactor is extremely difficult to remove because of the large quantity. Operators have resorted to dangerous procedures to facilitate its removal. For example, explosives have been used in an attempt to disintegrate the congealed mass into smaller pieces with only limited success. Also, chain saws have been used sometimes in conjunction with the explosives. The explosives have the problem that they are unpredictable and dangerous and often rupture the side of the reactor. The chainsaws are also dangerous and can result in serious accidents. Further, the blades of the saws require constant changing because of the gummy nature of the polyethylene. These problems are compounded because operators must physically enter the reactors This requires protective gear and a fresh air supply to protect the operators. After small particles agglomerate, the temperature remains high for a long period before cooling and solidifying. Thus, the whole operation is very time consuming, cutting into productivity and resulting in costly down time. It will be seen that there is a great need for a method and apparatus which efficiently removes the solidified mass of polyethylene from the Unipol reactor.
In the patent art, different mechanical apparatus have been suggested for working inside vessels. For example, U.S. Pat. No. 4,822,238 discloses a robotic arm positionable within a nuclear vessel by access through a small diameter opening and having a mounting tube supported within the vessel and mounting a plurality of arm sections for movement lengthwise of the mounting tube as well as for movement out of a window provided in the wall of the mounting tube. An end effector, such as a grinding head or welding element, at an operating end of the robotic arm, can be located and operated within the nuclear vessel through movement derived from six different axes of motion provided by mounting and drive connections between arm sections of the robotic arm.
U.S. Pat. No. 4,501,522 discloses a manipulator for remote handling in a hostile environment, the manipulator having a slave aim comprising an upper arm, a forearm and a jaw mechanism. The upper aim is pivotally suspended from a shoulder support and is telescopically extendible such that straight line motion of the forearm is readily obtained. In addition the forearm is arranged to pivot relative to the upper arm through the same angle as the upper arm but in the opposite sense such that the forearm maintains a constant orientation when the upper arm pivots.
U.S. Pat. No. 4,381,872 discloses a single hoist line operated clamshell bucket apparatus and method of use. The bucket apparatus is loaded in the usual manner at a first location for transporting bulk material to a second location where it is discharged. The bucket is discharged by sensing the presence of a preselected radio command signal that is controlled by the crane operator. The sensed radio signal operates a shut-off valve in a hydraulic controlled bucket latching system to release hydraulic fluid to permit discharge of the clamshell bucket.
U.S. Pat. No. 2,973,107 discloses a remote controlled manipulating apparatus for manipulating objects inside sealed chambers referred to as a telemanipulator which comprises a first group of control units which are outside a sealed chamber and which actuate at least one second group of corresponding operating units inside the chamber, and, according to the invention, is essentially characterized in this, that the connection between the above mentioned two groups of units is effected by means of at least one transmission system known per se and comprising on the one hand a first movable magnet disposed outside and against the wall of the chamber, which wall is made of non-magnetic material, and on the other hand a second magnet disposed inside and against the wall of the chamber.
U.S. Pat. No. 3,033,059 discloses a drive means for a remote control manipulator which comprises two main portions. The position containing the motor and speed reducer being designated as the drive portion, and a pivoted member or a member connected to the output shaft of the speed reducer and rotated thereby, which is the driven portion. In operation, with the drive portion fixed, it is the driven portion that is pivoted. If, however, the driven portion is fixed then the drive portion pivots. In each case the output shaft of the speed reducer serves as the pivotal axis and driving member for the pivoted joint.
U.S. Pat. No. 3,323,234 discloses an earth excavating apparatus having a detachable track that clamps to the periphery of the utility pole at a desired distance above the ground. A carriage, riding on the track, supports an extensible means which, in the embodiment chosen for illustration, takes the form of a hydraulic piston and cylinder. The opposite end of the piston and cylinder are connected to a novel scoop arrangement which is hydraulically operated and capable of scooping earth close to the surface of the pole.
U.S. Pat. No. 3,587,872 discloses a mechanical arm provided with a free end having gripping jaws thereon, and a pair of foot pedals connected by Bowden wires to the various elements to control swinging movement of the elements of the arm both vertically and horizontally and controlling opening and closing movements of the jaw members.
U.S. Pat. No. 3,737,059 discloses a multi-purpose bucket arrangement comprised of a first bucket portion and a second or cover bucket portion pivotally mounted thereon. A pair of hydraulic cylinders are operatively connected to the bucket portions to normally pivot the second bucket portion relative to the first one. The rod ends of the cylinders are pivotally connected to the distal end of a lever mounted on the pivotal connection for the bucket portions.
U.S. Pat. No. 4,220,170 discloses an apparatus for cleaning large tank interiors and other similar areas having a telescoping boom assembly attached to a base assembly with a positionable water nozzle assembly attached to one end of the boom assembly wherein the base member positioned in the tank interior is provided with a cage area housing the necessary control equipment and human operator.
U.S. Pat. No. 4,327,943 discloses a pair of bucket bowls pivotally suspended from a power head frame on the exterior of which is mounted a pair of hydraulic rams. Each of the hydraulic rams has one end pivotally connected to the power head frame and another end pivotally connected to a respective bucket bowl. The power head frame houses power components for actuating the cams and is formed of a horizontal deck, and vertical side and end bulkheads integrally formed to provide a watertight enclosure.
U.S. Pat. No. 4,545,723 discloses apparatus for adapting a general purpose end effector device to a special purpose end effector which includes an adapter bracket assembly which provides a mechanical and electrical interface between the end effector devices. The adapter bracket assembly includes an adapter connector post which interlocks with a diamond-shaped gripping channel formed in closed jaws and of the general purpose end effector. The angularly intersecting surfaces of the connector post and gripping channel prevent any relative movement therebetween. Containment webs constrain the outer finger plates of the general purpose jaws to prevent pitch motion. Electrical interface is provided by conical, self-aligning electrical connector components carried by respective ones of said end effectors.
U.S. Pat. No. 5,223,280 discloses an apparatus for taking residual solid matter out of an extrusion head of an extruder including a grasping device for grasping residual solid matter in the extrusion head, and a moving device for moving the grasping device between a grasping position in the extrusion head and an exhausting position on one side of the extrusion head. In order to remove the residual solid matter from the grasping device even if the solid matter firmly adheres to the grasping device, the apparatus further includes a disengaging device for urging the residual solid matter to disengage it from the grasping device when the grasping device has arrived at the exhausting position and released the residual solid matter. A control device is provided for controlling the grasping device, the moving device and the disengaging device.
In spite of these disclosures, there is still a great need for a device for removing solidified polyethylene from a Unipol reactor which efficiently and effectively removes the solidified material in the reactor with minimal downtime and without endangering the operators. Thus, the subject invention provides such a device.